This invention relates to internal combustion engines and, more particularly, to means for varying valve timing and intake and exhaust valve overlap in an internal combustion engine, particularly of the four stroke spark ignition type. In its more particular aspects the invention relates to valve actuating means variably yieldable as a function of engine speed to change with speed the effective lift and opening periods of intake and exhaust valves and thus vary the valve overlap with changes in engine speed. The invention also relates to a hydraulic valve tappet including means for varying its effective actuating length as a function of engine speed.
It is well known in the art that the fixed valve timing of internal combustion engines intended for variable speed operation, particularly of the four stroke cycle spark ignition type commonly used in automotive vehicles, represents a compromise as to engine operating efficiency and characteristics which could be improved by using mechanism for varying the valve timing with speed to obtain the proper timing for best operation at each engine operating speed. Among the valve operating characteristics, it is recognized that some variation with speed of overlap in the opening periods of the intake and exhaust valves is desirable. Thus, relatively small overlap or none at all would be provided for low engine speeds, while substantially greater valve overlap would be provided at increased engine speeds. In addition, it is desirable that the opening periods of individual valves be shortened at low engine speeds to correspond more closely with the extent of the intake or exhaust strokes of their respective pistons, while at higher speeds, better breathing results from increasing the times of opening of the valves with respect to crankshaft rotation.
The prior art is replete with mechanisms intended for use in obtaining variable valve timing and variable valve overlap to obtain some or all of the advantages possible for such mechanisms. In general, however, such mechanisms are not known to have been used in commercial engines. U.S. Pat. No. 3,385,274 Shunta et al. refers in its specification to a number of patents disclosing prior art mechanisms of this sort and itself discloses one arrangement for accomplishing variable valve lift, timing and overlap.